In the process of electrophotographic printing, a charge-retentive or photoconductive surface, also known as a photoreceptor, is charged to a substantially uniform potential, so as to sensitize the surface of the photoreceptor. The charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced, or else to a scanned laser image that is generated by the action of digital image data acting on a laser source. The scanning or exposing step records an electrostatic latent image on the photoreceptor corresponding to the informational areas in the document to be printed or copied. After the latent image is recorded on the photoreceptor, the latent image is developed by causing toner particles to adhere electrostatically to the charged areas forming the latent image. This developed or toner image on the photoreceptor is subsequently transferred to a sheet on which the desired image is to be printed. Finally, the toner on the sheet is heated to permanently fuse the toner image to the sheet.
One familiar type of development of an electrostatic image is called “two-component development.” Two-component developer material largely comprises toner particles interspersed with carrier particles. The carrier particles may be attracted magnetically and the toner particles adhere to the carrier particles through triboelectric forces. This two-component developer can be conveyed, by means such as a “magnetic roll,” to the electrostatic latent image, where toner particles become detached from the carrier particles and adhere to the electrostatic latent image.
In magnetic roll development systems, the carrier particles with the triboelectrically adhered toner particles are transported by the magnetic rolls through a development zone. The development zone is the area between the outside of a magnetic roll and the photoreceptor surface on which a latent image has been formed. Because the carrier particles are attracted to the magnetic roll, some of the toner particles are interposed between a carrier particle and the latent image on the photoreceptor. These toner particles are attracted to the latent image and transfer from the carrier particles to the latent image. The carrier particles are removed from the development zone as they continue to follow the rotating surface of the magnetic roll. The carrier particles then fall from the magnetic roll and return to the developer supply where they attract more toner particles and are reused in the development process. The carrier particles fall from the magnetic roll under the effects of gravity or are directed away from the roll surface by a magnetic field.
One type of carrier particle used in two-component developers is the semi-conductive carrier particle. Developers using this type of carrier particle are also capable of being used in magnetic roll systems that produce toner bearing substrates at speeds of up to approximately 200 pages per minute (ppm). Developers having semi-conductive carrier particles use a relatively thin layer of developer on the magnetic roll in the development zone. In these systems an AC electric waveform is applied to the magnetic roll to cause the developer to become electrically conductive during the development process. The electrically conductive developer increases the efficiency of development by preventing development field collapse due to countercharge left in the magnetic brush by the developed toner. A typical waveform applied to these systems is, for example, a square wave at a peak to peak amplitude of 1000 Volts and a frequency of 9 KHz. This waveform controls both the toner movement and the electric fields in the development zone. These systems may be run in a “with” mode, which means the magnetic roll surface runs in the same direction as the photoreceptor, or in an “against” mode, which means the magnetic roll runs in a direction that is the opposite direction in which the photoreceptor runs.
One embodiment of a two magnetic roll development station increases the time for developing the toner and provides an adequate supply of developer for good line detail, edges, and solids. The embodiment includes an upper magnetic developer roll and a lower magnetic developer roll with both developer rolls having a stationary core with at least one magnet and a sleeve that rotates about the stationary core. A motor coupled to the two magnetic developer rolls drives the rotating sleeves of the magnetic developer rolls in a direction that is against the rotational direction of a photoreceptor to which the two magnetic rolls deliver toner. The two magnetic developer rolls carry semi-conductive carrier particles and toner particles through a development zone formed by the magnetic developer rolls. A trim blade is mounted proximate the upper magnetic developer roll to form a trim gap of approximately 0.5 to approximately 0.75 mm.
This development station architecture has generally resulted in improved development for electrostatographic imaging machines. The two magnetic rollers arranged in the vertical architecture enable development of higher resolution images comprised of smaller toner dots on the photoreceptor. As the toner dots become smaller, the ratio of dot perimeter to the dot surface area becomes larger and variations in toner development for the dots become more apparent. At the toner dot sizes made possible by the vertical architecture noted above, toner development is adversely impacted by environmental conditions, particularly humidity. This adverse impact appears to arise from fluctuations in the electric fields generated by the AC waveform at the edge of the dots being developed on the photoreceptor. These fluctuations may result in dot formation variation that produces grainy half-tone images.
Known techniques for adjusting development station operations to compensate for changes in environmental conditions are not effective for adjusting the operation of the vertical roller architecture that is used for development of two component developer as discussed above. Attempts to scale these known operational parameters for use with the two vertical roller architecture described above have been frustrated with inconsistent results.
The development station and method discussed below improve toner dot edge development and stabilize toner dot size in a variety of environmental conditions.